THIS REMODELED WORK

I DEDICATE TO

HER

WHOSE AFFECTIONATE SYMPATHY

HAS GLADDENED MY LIFE

AND INSPIRED MY RIPER STUDIES.

PREFACE TO REVISED EDITION.

THE progress in chemistry during the ten years which have elapsed since this work was first published and stereotyped has been accompanied by no such revolution in its philosophy as the previous transition from the dualistic system of Berzelius to the unitary system of structural organic chemistry had involved. Nevertheless, there has been a constant advance, during which we have gained clearer conceptions and more comprehensive views of the fundamental principles of the science; and many of the accidental features which marked the transition period have disappeared. Meanwhile the distinction between elementary substances and materials consisting of isolated elementary atoms has become clear, and in making these last, alone, the elements of chemistry we have pushed our science, if not to its extreme limits, still one step further back; and in taking this step we have left behind many of the anomalies which previously encumbered our philosophy. Except in a very limited sense, the so-called elementary substances are now seen to be as truly com

pounded as any other substances, and it is manifest that their qualities must depend on molecular structure, or on the resulting dynamical relations, as well as on the fundamental attributes of the ultimate atoms. There is, therefore, no longer any reason for limiting the statement of the great fundamental law of definite proportions to the relations of elementary substance, and clearness of exposition is gained by giving to this statement the widest possible scope.

But unquestionably the most important advance in chemistry during the last decade has resulted from the study of the thermal changes accompanying chemical processes, which has proved that the law of the conservation of energy is a directing principle in chemistry as important as it is in physics. This study has developed an entirely new branch of our science called thermo-chemistry; and we now confidently look forward to a time in the near future when we shall be able to predict the order of phenomena in chemistry as fully as we now can in astronomy.

So important and fundamental have been the changes required by the recent progress that, in preparing this book for a new edition, the author has found it necessary to add a great deal of new material and in many places to rewrite the old, but he has endeavored to make the new edition, like the first, a popular exposition of the actual state of the science.

CAMBRIDGE, U. S. A., October 22, 1883.

PREFACE.

THE lectures now published were delivered before the Lowell Institute, in Boston, in the autumn of 1872. They aimed to present the modern theories of chemistry to an intelligent but not a professional audience, and to give to the philosophy of the science a logical consistency, by resting it on the aw of Avogadro. Since many of the audience had studied the elements of chemistry, as they were formerly taught under the dualistic system, it was also made an object to point out the chief characteristics by which the new chemistry differed from the old. The limitations of a course of popular lectures necessarily precluded a full presentation of the subject, and only the more prominent and less technical features of the new system were discussed. In writing out his notes for the press, the author has retained the lecture style, because it is so well adapted for the popular exposition of scientific subjects; but he